Layered products in which a gas barrier layer containing aluminum or aluminum oxide as a component is formed on a plastic film have been conventionally well-known. Such layered products are used as packaging materials for protecting articles (such as foods) which are susceptible to quality change induced by oxygen. In many cases, such a gas barrier layer is formed on a plastic film by a dry process such as physical vapor deposition (PVD) or chemical vapor deposition (CVD).
For example, aluminum-deposited films have light shielding properties as well as gas barrier properties and are typically used as packaging materials for dry foods.
Aluminum oxide-deposited films, which have transparency, are characterized by allowing visual recognition of contained substances and by enabling check for foreign matters with a metal detector and heating with a microwave oven. These films are thus used as packaging materials in a wide variety of applications such as retort food packaging.
For example, Patent Literature 1 discloses a composite structure having a gas barrier layer containing aluminum, the composite structure having a transparent gas barrier layer composed of aluminum atoms, oxygen atoms, and sulfur atoms. Patent Literature 1 discloses a method for forming the transparent gas barrier layer by reactive sputtering.
In addition, Patent Literature 2 discloses a composite structure having a gas barrier layer containing aluminum oxide, the composite structure having a transparent gas barrier layer composed of a reaction product of aluminum oxide particles and a phosphorus compound. Patent Literature 2 discloses a method for forming the gas barrier layer, in which a coating liquid containing aluminum oxide particles and a phosphorus compound is applied onto a plastic film, then dried and heat-treated.
Such conventional composite structures having a gas barrier layer have good initial gas barrier properties; however, when they are exposed to physical stresses such as deformation and impact, the gas barrier layer may suffer from defects such as cracks and pinholes which lead to deterioration in gas barrier properties.
Under such circumstances, Patent Literature 3 and Patent Literature 4 have proposed multilayer structures that not only have good gas barrier properties but also are capable of maintaining the gas barrier properties at a high level even when exposed to physical stresses such as deformation and impact.
However, when conventional multilayer structures as disclosed in Patent Literature 3 and Patent Literature 4 are used as packaging materials for retort foods, the multilayer structures may experience a decrease in interlayer adhesion and suffer from appearance defects such as delamination after retorting.
Thus, a gas-barrier multilayer structure that maintains good properties even after retorting has been demanded.